1. Field of the Disclosure
The invention relates to a transparent, particularly low-color, lithium aluminum silicate glass ceramic and to the use of such an LAS glass ceramic.
2. Description of Related Art
It is known that glasses of the Li2O—Al2O3—SiO2 system can transform into glass ceramics with high-quartz mixed crystals and/or keatite mixed crystals as main crystal phases.
A key property of these glass ceramics with high-quartz mixed crystals as the main crystal phase is the ability to produce materials that provide an extremely low heat expansion coefficient in a pre-specified temperature range. Usually the thermal expansion behavior is established so that the materials provide very low expansion, for the most part 0±0.3·10−6/K, in the region of their application temperatures. Thus, e.g., with application as substrate materials, wafer stages, or mirror supports for telescopes, thermal expansion in the region of room temperature is minimized.
For applications as fire-resistant glass, transparent fireplace windows or viewing panels or cooktops with colored underside coating, the zero thermal expansion in a temperature range between room temperature and approximately 700° C. is adjusted to the lowest possible values. For cooktops having gas and induction heating, maximum application temperatures of approximately 500° C. are sufficient. Current specifications for thermal expansion are approximately 0±0.15·10−6/K for cooktops and approximately 0±0.3·10−6/K for fireplace panels (for this, see Tables 3.3 and 3.4 in the book: “Low Thermal Expansion Glass Ceramics”, Second Edition, Editors Hans Bach, Dieter Krause, Springer Publishers, Berlin Heidelberg 2005, ISBN 3-540-24111-6).
Based on the low thermal expansion at their application temperatures, these glass ceramics possess an excellent resistance to differences in temperature and fluctuations in temperature, as well as dimensional constancy.
With application as cooktops, the transparent, low-color glass-ceramic panels for the most part are provided with an opaque color coating on the underside in order to prevent a view onto the technical components and to furnish a color effect. Recesses in the coating on the underside enable the introduction of colored and white displays, for the most part light diodes.
Transparent glass ceramics are distinguished from colored transparent glass ceramics, in which V2O5 is particularly added to the volume for coloring, in order to reduce light transmission to values below 5% and to prevent the view onto the technical components underneath the cooktop.
The large-scale production of transparent glass ceramics is conducted in several steps. First, the crystallizable initial glass made up of a mixture of shards and powder-form batch raw materials is melted and refined at temperatures usually between 1550 and 1700° C. Typically, arsenic oxide and sometimes antimony oxide are used as refining agents in the melt. For transparent glass ceramics with their comparatively high melting temperatures, arsenic oxide is the technically and economically most favorable refining agent with respect to good bubble qualities at conventional refining temperatures below 1700° C. In addition, arsenic oxide is advantageous for the transparency (high light transmission and little color) of the glass ceramics. For these compelling technical reasons, the transparent glass ceramics NEOCERAM® N-0 of the company Nippon Electric Glass, KERALITE® of the company Eurokera and ROBAX® of the company Schott AG, which are currently available on the market, are refined with arsenic oxide. Although these substances are solidly bound in the glass skeleton, they are a disadvantage from the aspects of safety and environmental protection. Thus, special precautionary measures must be taken in the recovery and processing of raw materials and because of their vaporization out of the melt. Because of this, numerous developmental attempts have been made to replace these materials, but these efforts could not be implemented previously for technical and economic reasons.
After melting and refining, the glass usually undergoes a hot forming by rolling or more recently also by floating, in order to produce plates or panels.
In a subsequent temperature process, the initial glass is converted into the glass-ceramic article by controlled crystallization. This ceramicizing takes place in a two-step temperature process, in which first, nuclei are produced by nucleation at a temperature between 680 and 810° C., usually from ZrO2/TiO2 mixed crystals. SnO2 can also participate in the nucleation. With subsequent increase in temperature, the high-quartz mixed crystals grow on these nuclei.
The structure of the glass ceramics is homogenized and the optical, physical, and chemical properties are established for the maximum production temperature. If desired, the high-quartz mixed crystals can subsequently still be converted into keatite mixed crystals. The transformation into keatite mixed crystals is produced with an increase in temperature in a range of approximately 970° C. to 1250° C. With the transformation, the thermal expansion coefficient of the glass ceramics increases and by further crystal growth, light scattering occurs, combined with a translucent to opaque appearance.
Absorption and scattering are the optical phenomena that must be mastered for economical production.
The brownish hue of transparent lithium aluminum silicate glass ceramics has different causes that are primarily based on absorption mechanisms and partially on scattering.
The coloring element Fe is contained as an impurity in the batch raw materials for the melts. The latter colors ionically as Fe3+ as well as via Fe/Ti color complexes. Due to the high cost of low-iron raw materials, it is not economical to reduce the Fe2O3 content to values of 100 ppm and thereunder.
Electronic transitions of color complexes, which absorb in the short-wave region of visible light and in which participates the component TiO2 that is effective for the nucleation, constitute the strongest absorption mechanism of transparent glass ceramics. The color complex arises due to the formation of adjacent Fe and Ti ions, between which occur electronic charge-transfer transitions.
With the use of SnO2 as an environmentally-friendly refining agent, the arising Sn/Ti complexes cause an additional absorption. The Fe/Ti color complexes lead to a red-brown coloring; the Sn/Ti color complexes lead to a yellow-brown coloring. The Sn/Ti color complexes color more intensely and this circumstance has previously made it difficult to substitute the refining agent arsenic oxide by SnO2 in the case of transparent glass ceramics.
The formation of the named color complexes largely occurs during ceramicizing.
In order to reduce the concentration of color complexes, it is advantageous to shorten the times for nucleation and crystallization. Opposing this is the fact that the shortening of the nucleation time leads to enhanced light scattering and the shortening of the crystallization time leads to irregularities in the article.
The effective nucleating agent TiO2 can only be replaced with disadvantages in the melt and in forming by the alternative nucleating agents ZrO2 and SnO2. This means that the desired low melting temperatures and short ceramicizing times lead to an enhanced coloring based on the color complexes, even without the visually disruptive scattering due to the TiO2 content required therefor.
Numerous developmental attempts have been made for producing environmentally-friendly, transparent glass ceramics without the use of the refining agents arsenic oxide and antimony oxide. These could not be implemented previously for technical and economic reasons. Transparency, i.e., high light transmission and little coloring without visually disruptive scattering could not be reconciled with favorable manufacturing conditions.
One approach involves compositions without the nucleating agent TiO2, which lead to disadvantages during production.
Thus, WO 2008 065167 A1 describes the production of environmentally-friendly, transparent glass ceramics without disruptive coloring. These glass ceramics avoid the addition of TiO2 as a nucleating agent and are based on a mixed nucleation by ZrO2 and SnO2. The ZrO2 contents necessary for sufficiently rapid nucleation are 2-5 wt. %, and the SnO2 contents are >0.4-3 wt. %. With these high contents of ZrO2 and SnO2 the melting of the batch is slowed down, the melting and forming temperatures are increased, and the devitrification resistance of the glass is adversely affected. During the forming, which occurs at viscosities around the processing temperature VA von 104 dPas, disruptive crystal phases containing Sn and Zr crystallize out. This leads to an unacceptable reduction in the strength of the glasses and the glass ceramics produced therefrom.
Another approach involves transparent glass ceramics without arsenic oxide and antimony oxide as refining agents with small contents of TiO2, but which also require higher contents of SnO2 and ZrO2 as nucleating agents. In WO 2008 065166A1, TiO2 is limited to 0.3-<1.6 wt. %. Contents of SnO2 from 0.25-1.2 wt. % and ZrO2 from >2-3.8 wt. % are required. These high contents are accompanied by the described disadvantages in the melt and in forming as well as a deficient devitrification resistance.
The documents JP 11-228180 A2 and JP 11-228181 A2 describe environmentally-friendly compositions of transparent glass ceramics. In order to obtain sufficient bubble qualities without using arsenic oxide as a refining agent, the glass ceramics contain a combination of the refining agents SnO2 and Cl at 0.1-2 wt. %. The physical decoloring agent Nd2O3 is not used, so that the Sn/Ti color complex is fully brought to bear. In particular, the high SnO2 contents shown in the embodiment examples are very harmful for the devitrification resistance. The documents do not provide any indications of how the SnO2 content must be limited so as to assure sufficient devitrification resistance. In addition, these documents do not provide any indication for optimizing the manufacturing properties by the selection of the components CaO and SrO and the adjustment of crystal composition and composition of the residual glassy phase by the ratios of the divalent components MgO, ZnO as well as CaO, SrO and BaO.
The physical decoloring of transparent glass ceramics by additions of Nd2O3 and CoO, which absorb in the longer-wave red spectral region is disclosed in EP 1837312 B1. The document preferably describes compositions refined with arsenic oxide. In addition to the use of arsenic oxide, the use of 0.1-0.4 wt. SnO2 in combination with high-temperature refining over 1700° C. is also disclosed as an environmentally-friendly refining agent. This document does not provide any indications as to how the composition must be created in order to obtain particularly favorable manufacturing conditions, i.e., low melting and low forming temperatures.
There is thus a need for decreasing the melting and forming temperatures without disadvantages for the rate of ceramicizing, since these are of crucial importance for energy efficiency and economical production. In addition, this step does not provide any indications for optimizing the devitrification resistance and transparency by the selection of the components CaO, SrO and BaO. The establishing of crystal composition and composition of the residual glassy phase for improving the transparency with short ceramicizing times by means of the ratios of the divalent components MgO, ZnO as well as CaO, SrO and BaO is not described.